1. Field of the Invention
The present invention relates to a device for generating and sending ultrasonic waves.
2. Description of Related Art
An ultrasonic wave generating device is used for an ultrasonic sensor mounted to a vehicle, e.g., automobile. The sensor sends ultrasonic waves from the device to an object. The object reflects the waves, and a receiving element in the sensor receives the reflected waves. Thus, a position or a distance for the object around the automobile can be measured. Further, a two-dimensional shape or a three-dimensional shape of the object can be measured. That is, the ultrasonic sensor is used for a safe driving by monitoring surroundings of the automobile.
For example, an automatic parking supporting system using an ultrasonic sensor is practically used. In the system, a back-sonar is used for detecting a human or an obstacle existing in the back of an automobile. The ultrasonic sensor is mounted to a rear part of the automobile, and receives ultrasonic waves reflected by the human or the obstacle. Thus, a back side collision with the human or the obstacle can be reduced.
JP-A-11-300274 discloses an ultrasonic wave generating device used in an ultrasonic sensor. The device is formed by layering a heating membrane on a substrate through a heat insulation layer with a micro electro mechanical system (MEMS) technology. The heating membrane is electrically driven, and generates a temperature variation corresponding to a period of an ultrasonic wave in air layer on the membrane. Thus, the device can thermally generate ultrasonic waves.
However, when the device is mounted to an automobile in an exposed state, water drops or dusts may adhere onto a surface of the device. In this case, efficiency for generating ultrasonic waves may be decreased, or the device may not generate ultrasonic waves. Then, a distance to an object may not accurately be measured. Further, the device may be damaged by a load of an external force, e.g., a collision with a small stone.
By contrast, when a protecting portion for reducing the damage is disposed in front of the device through air space, ultrasonic waves may be attenuated, because an impedance difference between the air space and the protecting portion is large. In this case, ultrasonic waves may not be transmitted to a farther position. Thus, a mounting state of the device is limited.